Sulfur oxides contained in the waste gases from boilers and other industrial furnaces and the like are the sources of air pollution and even a small amount of them present in the atmosphere will be hazardous to the human body. It is, therefore, necessary to prevent such sulfur oxides from being dispersed into the atmosphere. For this purpose, there must be provided an apparatus which is not only capable of removing such sulfur oxides from industrial waste gases, but one that is also practical and economical for industrial applications.
On the other hand, to meet the scarcity of industrial and domestic fresh water, apparatus for the distillation of sea water have recently been used, particularly employing multistage flash evaporators. The type of sea water distillation employing heat, such as the multistage flash evaporator, requires the pretreatment of the sea water to prevent the production of scales in the distillation apparatus. The most useful pretreatment of the sea water for this purpose relates to PH control, in which the PH of the sea water is controlled by injecting acids into the sea water.
There is known a multistage flash evaporator suitable for large capacity and low cost production. The apparatus uses the principal whereby the pressure of heated sea water is reduced at saturation pressure, so that the sea water starts to boil and instantaneously evaporate. With respect to structure, materials, the prevention of scale formation and the entire system, there has been considerable research and development that is quite advanced. Supplementary sea water or makeup sea water to the multistage flash evaporator usually requires the pretreatment of the sea water for preventing the deposition of scales on a surface of tubes of the heat exchanger and corrosion of the structural materials. In such pretreatments for the prevention of scale deposition by PH control, the addition of chemicals has been regarded as a practical and useful means for the prevention of soft scale formation. The injection of acids however has brought about the rising cost of distillation.
In order to remove sulfur oxides from waste gas, there has been proposed a dry method employing powdered or granular absorbents, reagents and catalizers, and a wet method using alkaline solutions or calcium hydroxide. The dry method has been found to be quite costly and employs large size apparatus for the regeneration of the active carbons or manganese. The active carbon method still remains a problem with respect to the life of the active carbons and the manganese method contains a defect such that a secondary pollution is caused by the scattering of the powdered manganese. The wet method brings about a remarkable temperature fall and consumes a lot of chemical materials as neutralizers. These prior desulfurizing methods have produced extensive by-products and have required various apparatus in order to effectively utilize the by-products and to convert them into harmless material that may be disposed of.